Photomultiplier tubes are used to detect photons and exhibit a very high gain factor. Typical gain factors of photomultiplier tubes may exceed 1,000,000. A photomultiplier tube includes a photocathode that receives photons and in response emits electrons, multiple dynodes, each dynode is arranged to receive electrons from a previous dynode and emit more electrons that it received, and an anode arranged to receive the electrons emitted from a last dynode and to output an electrical detection signal.
Typically, photo-detector devices built with photomultiplier tubes have a biasing circuit that provides biasing voltages to the photocathode, the anode and the dynodes by a chain of resistors connected in series. Capacitors may be connected in parallel to the resistors to improve output linearity at short light pulses.
When working at high gains the current drained by each dynode (equivalent to the difference between the electrons received by a dynode and the electrons emitted by the dynode) dramatically changes between the dynodes. Thus, “low current” dynodes—dynodes that follow the photocathode drain currents that are smaller (by few order of magnitudes) than the current drained by “high current” dynodes—dynodes that precede the anode.
U.S. Pat. No. 7,005,625 of Mitchell, titled “Low power stabilized voltage divider network” makes a clear distinction between (a) the photocathode and “low current” dynodes that follow the photocathode and (b) anode and “high current” dynodes that precede the anode. The former are biased by resistors while the latter are biases by transistors. This patent is incorporated herein by reference.
There is a growing need to provide an efficient photo detector built with a photomultiplier tube that will be able to operate at wide gain range and wide incident light intensity range.